The present invention relates to a scanning lens for a scanning optical system used for a laser beam printer or the like, and particularly to a scanning lens that functions for two different wavelengths.
A scanning optical system is used for converging a scanning beam on a surface to be scanned. The scanning beam is typically configured such that a beam emitted by a laser source is deflected using a deflector such as a polygonal mirror, and the deflected beam is converged by an fxcex8 lens to form a moving beam spot on the surface such as a circumferential surface of a photoconductive drum. As the polygonal mirror rotates, the beam spot formed on the photoconductive drum moves in one direction, which will be referred to as a main scanning direction. When the beam spots moves in the main scanning direction, the beam is ON/OFF modulated in accordance with an image to be formed. While the beam spot moves in the main scanning direction, the photoconductive drum is rotated (i.e., the surface is moved in an auxiliary scanning direction that is perpendicular to the main scanning direction). With the above configuration, a two-dimensional latent image is formed on the surface of the photoconductive drum.
A conventional scanning lens is designed to exhibit the best performance at a predetermined wavelength. If the wavelength of a beam actually used is different from the designed wavelength, its imaging performance is lowered.
Japanese Patent provisional publication HEI 10-197820 discloses a scanning optical system provided with a diffractive lens structure that compensates for a lateral chromatic aberration. With this scanning optical system, an effect of difference in wavelengths of laser source due to a manufacturing error of laser diodes is suppressed.
Japanese Patent provisional publication HEI 9-105877 teaches a scanning optical system provided with a diffractive lens structure that compensates for a longitudinal chromatic aberration. This scanning optical system is designed for a multi-beam scanning system, and an effect of a difference in wavelength between a plurality of beams is compensated.
In the JP provisional publication HEI 10-197820, the scanning optical system is designed to compensate for a deviation of a wavelength with respect a predetermined design wavelength. However, there is no disclosure regarding a condition required for a diffractive lens employed therein to compensate for a lateral chromatic aberration for two different wavelengths.
In JP provisional publication HEI 9-105877, the lateral chromatic aberration is not compensated for, therefore, and for beams having different wavelengths, scanning widths of scanning lines are different.
The present invention provides a scanning optical system that exhibits excellent imaging performance for two design wavelengths.
To provide the above advantage, a scanning lens which converges a beam deflected by a deflector on a surface to be scanned, is provided with at least one refractive lens, and a diffractive lens structure formed on at least one surface of the at least one refractive lens. The scanning lens is configured to satisfy the following condition:
9.0 less than fd/f less than 17.0, 
where fd is a focal length of the at least one diffractive lens structure, f is a focal length of the scanning lens as a whole including the at least one diffractive lens structure and the at least one refractive lens. Further, the scanning lens is configured so that lateral chromatic aberration is compensated for both of a first wavelength and a second wavelength which is different from the first wavelength.
According to the invention, it becomes possible that the same scanning lens is used for both of the first wavelength and second wavelength. That is, at least for two design wavelengths, the same scanning lens can be used. Therefore, designing cost and manufacturing cost can be suppressed.
Optionally, the first wavelength is included in a red range, and the second wavelength is included in an infrared range. Further, a difference between the first and second wavelengths is substantially 100 nm or more.
Further optionally, Abbe number xcexdd of the refractive lens, on which the at least one diffractive lens structure is formed, is greater than 50.
Furthermore, a blaze wavelength of the at least one diffractive lens structure xcexB satisfies the following condition:
xcex1 less than xcexB less than (xcex1+xcex2)/2, 
where xcex1 is the first wavelength, and xcex2 is the second wavelength.